Various studies have used different classification systems to evaluate and quantify the AC angle, the ‘gold standards’ of which are the gonioscopy in combination with a Goldmann contact lens examination, and the classification system introduced by Shaffer.14
However, in this system, the examiner numerically estimates the AC angle based on anatomic landmarks, which is subjective and highly dependent on the examiner’s judgment and experience.
Previous reports have described several parameters to quantify normal AC angle using OCT and ultrasound biomicroscopy (UBM):15–17
- Angle opening distance at 500 µm (AOD 500) is defined as the linear distance between the trabecular meshwork and the iris at 500 µm anterior to the scleral spur.
- Angle recess area at 500 µm and 750 µm (ARA 500 and ARA 750) is the triangular area formed by theAOD500 orAOD750 (the base), the angle recess (the apex), the iris surface and the inner corneo-scleral wall (the sides of triangle; ). Theoretically, ARA is a better measurement parameter than AOD, because it accounts for the whole contour of the iris surface, rather than measuring at a single point on the iris.
(a) Visante optical coherence tomography image of an anterior chamber angle: angle open distance (AOD) and angle recess area (ARA); (b) trabeculo-iris space area (TISA). Does not include the area posterior to the scleral spur.
- Trabeculo-iris space area at 500 µm and 750 µm (TISA 500 and TISA 750) is a trapezoidal area with the following boundaries: the anterior AOD 500 or AOD 750, a posterior line drawn from the scleral spur perpendicular to the plane of the inner scleral wall to the opposing iris, the superior inner corneo-scleral wall and the inferior iris surface. This parameter may better represent the actual filtering area compared with ARA, because TISA excludes the non-filtering region behind the scleral spur ().
- AC angle in degrees, in which the angle recess forms the apex of the AC angle, and the two arms are formed by drawing lines through the points defining the AOD 500 ().
Anterior chamber angle, in degrees (Visante optical coherence tomography).
Goniometry studies quantified a normal eye as one in which the open angle has a ~329-µ1m AOD 500 and a ~28° AC angle.18
In narrow and closed angles, the AOD 500 is <210 µm and the AC angle is <18°. Radhakrishnan et al.
in a study of 31 eyes found that an AOD 500 cut-off value of 190 µm was 100% sensitive and 87.5% specific in detecting an occludable angle.16
Thus, this criterion suggests that an AOD 500 of <190 µm is occludable.
In recent years, several studies have used anterior segment OCT to visualize AC angle structures.2,13,19,20
The non-contact nature of OCT permits comfortable and safe exams and, more importantly, sidesteps mechanical distortion of the angle and improves the reliability of the angle depth measurement. This technique provides high-resolution cross-sectional images, and it yields reproducible and accurate measurements of the AC.13,21
In October 2005, Food and Drug Administration approved the commercially available Visante OCT for AC angle imaging. This apparatus uses a 1300-nm wavelength and acquires images at a rate of eight frames (2000 A-scans) per second with a transverse resolution of 60 µm and an axial resolution of 10–20 µm. Recent studies with Visante OCT determined that this apparatus could pinpoint scleral spur locations in 72% of eye images.22,23
Its location on anterior segment OCT images was less detectable in quadrants with a closed angle during gonioscopy, or in images obtained in areas that were superior or inferior of the nasal and temporal quadrants. This inability to detect scleral spur may hamper quantitative analysis of AC angle parameters that are dependent on spur location, particularly in the superior and inferior quadrants. Indeed, in a second study, the same authors could not classify the AC angle in 16% of the eyes because of poor image quality or poor scleral spur definition.
A recently approved FD-OCT (September 2007), RTVue, exhibits an 830-nm wavelength, 26000 A-scans per second and an axial resolution of 5 µm, rendering it able to visualize scleral spur and other small anatomic details of the angle chamber (such as the Schlemm’s canal, the Schwalbe’s line and the trabecular meshwork) with higher resolution and image quality than the Visante OCT. and show TD-OCT and FD-OCT angle scans, respectively. In both images, the structures in the posterior chamber are not well delineated because of attenuation of the OCT light beam by the iris pigment epithelium.
Time-domain optical coherence tomography (Visante) angle scan.
Fourier-domain optical coherence tomography (RTVue) angle scan.